The work of specifying the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) consisting of the Long Term Evolution (LTE) and System Architecture Evolution (SAE) concepts is currently ongoing within the 3rd Generation Partnership Project (3GPP).
One important focus area in E-UTRAN standardization work is to ensure that the new network is simple to deploy and cost efficient to operate. The vision is that the new system shall be self-optimizing and self-configuring in as many aspects as possible, see NGMN, “Operator Use Cases related to Self Organising Networks,” ver. 1.53, 2007-04-16 and 3GPP TR 32.816, Study on Management of E-UTRAN and SAE. One aspect that benefits from self-optimization and self-configuration is the management of the random access channel (RACH). Also Ericsson “RACH optimization function” 3GPP draft R3-090825 describes optimization.
The architecture of the LTE system is shown in FIG. 1. FIG. 1 illustrates the LTE architecture showing logical interfaces between eNBs (X2) and between eNB and MME/S-GW (S1). In LTE the downlink is based on orthogonal frequency division multiplexing (OFDM) while the uplink is based on a single carrier modulation method known as discrete Fourier transform spread OFDM (DFT-S-OFDM), see 3GPP TR 36.300, Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2, V8.2.0.
In the following sections the random access procedure for LTE as it is currently defined by 3GPP is outlined.
Some E-UTRA Physical Layer Details
Downlink and uplink transmissions are in LTE organized into radio frames with 10 ms duration. Two radio frame structures are supported:                Type 1, applicable to Frequency Division Duplex (FDD),        Type 2, applicable to Time Division Duplex (TDD).        
Frame structure Type 1 is illustrated in FIG. 2. Each 10 ms radio frame is divided into ten equally sized sub-frames. Each sub-frame consists of two equally sized slots. For FDD, 10 subframes are available for downlink transmission and 10 subframes are available for uplink transmissions in each 10 ms interval. Uplink and downlink transmissions are separated in the frequency domain.
Frame structure Type 2 is illustrated in FIG. 3. Each 10 ms radio frame consists of two half-frames of 5 ms each. Each half-frame consists of eight slots of length 0.5 ms and three special fields: DwPTS, Guard Period (GP) and UpPTS. The length of DwPTS and UpPTS is configurable subject to the total length of DwPTS, GP and UpPTS being equal to 1 ms. Both 5 ms and 10 ms switch-point periodicity are supported. Subframe 1 in all configurations and subframe 6 in configuration with 5 ms switch-point periodicity consist of DwPTS, GP and UpPTS. Subframe 6 in configuration with 10 ms switch-point periodicity consists of DwPTS only. All other subframes consist of two equally sized slots.
For TDD, GP is reserved for downlink to uplink transition. Other Subframes/Fields are assigned for either downlink or uplink transmission. Uplink and downlink transmissions are separated in the time domain.
The physical channels of E-UTRA are:
Physical Broadcast Channel (PBCH)
                The coded BCH transport block is mapped to four subframes within a 40 ms interval;        40 ms timing is blindly detected, i.e. there is no explicit signalling indicating 40 ms timing;        Each subframe is assumed to be self-decodable, i.e. the BCH can be decoded from a single reception, assuming sufficiently good channel conditions.Physical Downlink Control Channel (PDCCH)        Informs the User Equipment UE about the resource allocation of PCH and DL-SCH, and Hybrid Automatic Repeat Request (ARQ) information related to DL-SCH;        Carries the uplink scheduling grant.Physical Downlink Shared Channel (PDSCH)        Carries the DL-SCH and PCH.Physical Uplink Control Channel (PUCCH)        Carries Hybrid ARQ Acknowledgement/Negative Acknowledgements (ACK/NAKs) in response to downlink transmission;        Carries Scheduling Request (SR);        Carries Channel Quality Indicator (CQI) reports.Physical Uplink Shared Channel (PUSCH)        Carries the UL-SCH.Physical Random Access Channel (PRACH)        Carries the random access preamble.Random Access Procedure in LTE        
During initial access, the UE seeks access to the network in order to register and commence services. The random access (RA) serves as an uplink control procedure to enable the UE to access the network. Since the initial access attempt cannot be scheduled by the network, the RA procedure is by definition contention based. Collisions may occur and an appropriate contention-resolution scheme needs to be implemented. Including user data on the contention-based uplink is not spectrally efficient due to the need for guard periods and retransmissions. Therefore, it has been decided to separate the transmission of the random access burst (preamble), whose purpose is to obtain uplink synchronization, from the transmission of user data.
The RA procedure serves two main purposes:                It lets the UE align its Up-Link (UL) timing to that expected by the base station eNode B in order to minimize interfering with other UEs transmissions. UL time alignment is a requirement in E-UTRAN before data transmissions can commence.        It provides a means for the UE to notify the network of its presence and enables the eNode B to give the UE initial access to the system.        
In addition to the usage during initial access, the RA will also be used when the UE has lost the uplink synchronization or when the UE is in an idle or a low-power mode or when the UE has no dedicated scheduling request resource on the physical uplink control channel (PUCCH).
Prior to sending a preamble, the UE is to synchronize to the downlink transmissions and read the BCH. The BCH will reveal, among other parameters, where the RA time slots are located, which frequency bands can be used and which preambles (sequences) are available. LTE provides for 64 preambles for each cell. The set of preambles allocated to a cell shall not overlap with the set of preambles allocated to a neighboring cell as this could cause errors and ambiguities in preamble detection.
The basic RA Procedure is a four-phase procedure as outlined in FIG. 4:                Phase 1 consists of transmission of a random access preamble, allowing the Node B to estimate the transmission timing of the UE. Uplink synchronization is necessary as the UE otherwise cannot transmit any uplink data.        Phase 2 consists of the network transmitting a timing advance command to correct the uplink timing, based on the timing of arrival measurement in the first step. In addition to establishing uplink synchronization, the second step also assigns uplink resources and temporary identifier to the UE to be used in the third step in the random access procedure.        Phase 3 consists of signaling from the UE to the network using the UL-SCH similar to normal scheduled data. A primary function of this message is to uniquely identify the UE. The exact content of this signaling depends on the state of the UE, e.g., whether it is previously known to the network or not.        Phase 4, the final phase, is responsible for contention resolution in case multiple UEs tried to access the system on the same resource.        
For cases where the network knows, in advance, that a particular UE will perform a Random Access Procedure to acquire uplink synchronization, e.g., during Handover (HO), a dedicated preamble is reserved and assigned to the UE under consideration. When the UE transmits the dedicated preamble in Phase 1, the network knows to which UE this preamble was assigned and can already at the time of detection of this preamble determine the identity of the UE. Thus, in this scenario, no contention resolution is needed and the delay before data transmission can be resumed is reduced.
Phase 1—Random Access Preamble
Prior to sending a preamble, the UE shall synchronize to the downlink transmissions and read the BCH. The BCH will reveal, e.g., where the RA time slots are located, which frequency bands can be used and which preambles (sequences) are available.
At the next RA slot, the UE will send the preamble. The preamble sequence implicitly includes a random ID which identifies the UE. LTE provides for each cell 64 such random IDs and thus 64 preambles.
If multiple RA frequency bands have been defined, the UE randomly selects one of them. The group of sequences allocated to a cell is partitioned into two subgroups. By selecting a preamble sequence from a specific subgroup, the UE can give a single-bit indication of its resource requirement and/or link quality. The particular sequence used for the preamble is randomly selected within the desired subgroup. This sequence implicitly contains a random ID which serves as a UE identifier.
The eNode B estimates the UL timing of the UE based on the timing of the received preamble.
Phase 2—Random Access Response
After the preamble transmission, the UE waits for a RA Response message on the DL-SCH, the DL assignment which is indicated on the Layer 1/Layer 2 (L1/L2) control channel (DDCCH).
The RA Response message is transmitted semi-synchronously (i.e. within a window) to the reception of the RA Preamble in order to allow the scheduler more flexibility. The RA Response contains:                the same random UE identity as present in the preamble;        a time alignment message to provide the proper uplink timing to the UE;        a temporary Radio Network Temporary Identifier (RNTI) which is unique for the particular RA resource (time and channel) used in Phase 1. For initial access, the temporary RNTI shall be used for Phases 3 and 4;        an Up-Link (UL) resource grant for transmission on UL-SCH in Phase 3.        
If no RA Response message has been received after a certain time following the preamble transmission, the UE shall send a new preamble at the next RA time slot. In some cases the eNodeB may indicate an overload (too many preambles detected) and instruct the UE to wait for some time before attempting random access again. For the new attempt, the UE shall select new, random parameters for the preamble sequence and the non-synchronized RA frequency band. Furthermore, the UE will increase the power level of the preamble to obtain a power ramping procedure similar as used in Wide band Code Division Multiple Access (WCDMA) systems.
Phase 3—First Scheduled UL Transmission
In Phase 3, the UE provides the network with a unique identifier in the message it transmits on UL-SCH according to the grant contained in the RA Response. The type of UE identifier, e.g. C-RNTI, TMSI, IMSI or IMEI, depends on which extent the UE is already known in the network. In case of initial access, the message is a Radio Resource Control (RRC) Connection Request message.
In case of non-initial access, i.e. when the UE is already RRC_CONNECTED, the UE identifier is the C-RNTI and is signaled by the Media Access Control (MAC) layer. The transmission uses Hybrid Automatic Repeat Request (HARQ).
Phase 4—Contention Resolution
The purpose of the fourth phase is to resolve contention. Note that, from the second step, multiple UEs performing simultaneously random access attempts using the same preamble listen to the same response message and therefore have the same temporary identifier. Hence, in the fourth phase, the eNode B echoes the UE identity provided by the UE in Phase 3. Only a terminal which finds a match between the identity received in the fourth step and the identity transmitted as part of the third step will declare the random access procedure successful. This terminal will also transmit a hybrid ARQ acknowledge in the uplink. For non-initial access, i.e. when the UE is already RRC_CONNECTED, the UE identity is reflected on the L1/L2 control channel. If the UE has not yet been assigned a C-RNTI, the temporary identity from the second step is promoted to the C-RNTI, otherwise the UE keeps its already assigned C-RNTI.
UEs which do not find a match between the identity received in Phase 4 and the respective identity transmitted as part of Phase 3 are considered to have failed the random access procedure and need to restart the random access procedure with Phase 1; selecting new random parameters for the preamble sequence and the RA frequency band. No hybrid ARQ feedback is transmitted from these terminals.
Contention-Free Random Access Procedure
For cases where the network knows, in advance, that a particular UE will perform a Random Access Procedure to acquire uplink synchronization, a dedicated preamble is reserved and assigned to the UE under consideration. Dedicated Preamble assignment for HO is handled by RRC whereas preamble assignment for DL data arrival is handled by MAC. When the UE transmits the dedicated preamble in Phase 1, the network knows to which UE this preamble was assigned and can already at the time of detection of this preamble determine the identity of the UE. Thus no contention resolution is needed and the delay before data transmission can be resumed is reduced.
Random Access Back-Off Procedure
For the event of Random Access overload, a Random Access Back-Off procedure is supported. This procedure prevents immediate new Random Access attempts which would only worsen a collision situation.
In case of an overload, the eNodeB signals through the Random Access Response message a backoff indicator B. The UE that does not receive a random access response message that includes the transmitted preamble will wait a time which is uniformly distributed between 0 and B before attempting random access gain.
Random Access Channel Physical Resource
A single RA opportunity consists of a time slot and a fixed bandwidth. The RA time slot length TRA shall accommodate the preamble sent by the UE and the required guard period (GP) to take into account the unknown uplink timing, see FIG. 5. The timing misalignment amounts to 6.7 μs/km. 3GPP has decided for a minimum TRA of 1 ms. Here the preamble length is then 800 μs plus a cyclic prefix of around 102.5 μs. A guard time of 97.5 μs suffices for a cell radii up to 15 km. Larger guard periods and cyclic prefix are needed to accommodate timing uncertainties from cells larger than 15 km. Such large cells may also require longer preambles to increase the received energy. In order to support RA under various cell conditions RAN1 has defined additionally 3 RA preamble formats which require a TRA of 2 ms or even 3 ms. These larger slots are created by the eNode B by not scheduling traffic in the consecutive sub-frame(s). Those extended preambles contain repetitions of the 800 μs long part and/or a longer cyclic prefix.
For TDD an additional “short” RA is defined. The short RA preamble only spans 133 μs. Because of this very short duration the preamble will most likely not contain a cyclic prefix but a technique called overlap-and-add will be used to enable frequency-domain processing, see FIG. 5. At the time of writing many details regarding applicability and performance of this short RA are still open.
According to 3GPP, the bandwidth of a RA opportunity is 1.08 MHz (6 RB). The effective bandwidth utilized by the RA preamble is 1.05 MHz leaving small spectral guard bands on each side. This is necessary since RA and regular uplink data are separated in frequency-domain but are not completely orthogonal.
For FDD systems, RA opportunities do not occur simultaneously in different frequency bands but are separated in time. This spreads out processing load in the RA receiver. 3GPP defines RA configurations determining how often RA opportunities occur. In total 16 such configurations are defined, ranging from one RA opportunity every 20 ms (very low RA load) to one every 1 ms (very high RA load).
In TDD not all sub-frames are down link (DL) sub-frames reducing sub-frames that can be allocated to RA. To also in TDD configurations provide for high RA loads, multiple RA opportunities can be scheduled in a single sub-frame.
For FDD RA opportunities are restricted to the outermost 6 RBs of the physical uplink shared channel at the band edges.
The TDMA/FDMA structure of the RA opportunities in FDD is visualized in FIG. 6. Here only one 1.08 MHz band is allocated to RA at each time whereas several bands are possible in case of TDD. The RA opportunities always occur at the band edges of the physical uplink shared channel directly adjacent to the physical uplink control channel. In the example shown in FIG. 6, 3 RA opportunities with 1 ms length exist in each frame.
Preamble Format
FIG. 7a shows the detailed timing of the basic random-access preamble. The preamble is prefixed with a cyclic prefix (CP) to enable simple frequency domain processing. Its length is in the order of TGP+TDS=97.5+5 μs=102.5 μs, where TDS corresponds to the maximum delay spread and TGP corresponds to the maximum round trip time. The CP insures that the received signal is always circular (after removing the CP in the RA receiver) and thus can be processed by Fast Fourier Transformations (FFTs). Therefore, the “active” random-access preamble duration is 1000 μs−2·TGP−TDS=800 μs. The RA subcarrier spacing is 1/800 μs=1250 Hz.
FIGS. 7b to 7d show the extended preamble formats. Format 1 has an extended CP and is suited for cell radii up to approximately 100 km. However, since no repetition occurs this format is only suited for environments with good propagation conditions. Format 2 contains a repeated main preamble and a cyclic prefix of approximately 200 μs. With a RA opportunity length of 2 ms the remaining guard period is also approximately 200 μs. This format supports cell radii of up to approximately 30 km. Format 3 also contains a repeated main preamble and an extended CP. Using a RA opportunity length of 3 ms this format supports cell radii of up to approximately 100 km. In opposite to format 1 format 3 contains a repeated preamble and is therefore better suited for environments with bad propagation conditions.
Root Sequences in LTE
Preambles in LTE are based on Zadoff-Chu sequences. A Zadoff-Chu sequence of length N can be expressed, in the frequency domain, as
            X      ZC              (        u        )              ⁡          (      k      )        =      ⅇ                  -        j            ⁢                          ⁢      π      ⁢                          ⁢      u      ⁢                        k          ·                      (                          k              +              1                        )                          N            where u is the index of the Zadoff-Chu sequence within the set of Zadoff-Chu sequences of length N=838. Out of one Zadoff-Chu sequence—in the following also denoted root sequence—multiple preamble sequences can be derived by cyclic shifting, were a shift is given by su,v(n)=su(n−νNCS mod N) where su is the inverse discrete Fourier transform (IDFT) of XZC(u)(k).
Due to the ideal ACF of Zadoff-Chu sequence multiple mutually orthogonal sequences can be derived from a single root sequence by cyclic shifting one root sequence multiple times the maximum allowed round trip time plus delay spread in time-domain. The number of shifts and as such the number of preambles that can be derived from a root sequence depends on, e.g., the coverage of the cell.
Preamble Detection
A receiver at the eNodeB correlates the received signal with all the root sequences (Zadoff-Chu sequences) allocated to the eNodeB, see FIG. 8. If the correlation (height of the correlation peak) due to a preamble is higher than the detection threshold, then the preamble is detected. However, if the correlation is lower than the detection threshold then the preamble is not detected. In the latter case there is a detection miss. The detection miss probability is the probability that the correlation between the root sequence and the received signal is less than the detection threshold when in fact a preamble was sent (i.e., there is a miss detection).
RACH Power Control
Power control for RACH in LTE is as follows:PRACH(N)=min{PMAX,PO_RACH+PL+(N−1)ΔRACHΔPreamble}.where                PRACH is the preamble transmit power,        N=1, 2, 3, . . . is the RACH attempt number        PMAX is the maximum UE power,        PO_RACH is a 4-bit cell specific parameter signaled via BCCH with a granularity of 2 dB (difference in maximum and minimum PO_RACH is 30 dB)        PL is the path loss estimated by the UE        ΔRACH is the power ramping step signaled via BCCH and represented by 2 bits (4 levels) with a granularity of 2 dB        ΔPreamble is a preamble-based offset (format 0-3), see Section 0 above.        
Note that RACH attempts N=2, 3, 4, . . . includes retransmissions where                no RA Response message has been received by the UE (see FIG. 4),        the RA Response message is intended for another preamble (UE)        the contention resolution has failed and the UE has to try random access again.        
In essence, the UE will increase its transmission power until network access is granted. There is typically an upper bound on the number of retransmissions and, thus, number of power increases.
Performance Indicators Related to Random Access
The UE performs a power ramping procedure, where the UE increases its power for the subsequent preamble transmission if the UE is not granted access due to a preamble detection miss or contention. The desired performance of RACH may be specified in terms of the access probability AP(m), which is the probability that the UE has access after a certain random access attempt number m=1, 2, 3 . . . . For example, we may require that the access probability should be greater than 0.8 and 0.95 at attempts 1 and 3, respectively, i.e. AP(1)>0.8 and AP(3)>0.95.
Alternatively, the desired performance may be specified in terms of the access delay, which is the delay from the initial random access attempt until access is granted. Similar to above, the RACH performance may be specified in terms of the access delay probability ADP(δ), which is the probability that the access delay is less than. It is possible to specify the desired performance in terms of two access delay requirements, e.g., ADP(20 ms)<0.8 and ADP(40 ms)<0.95.
Alternatively, the requirements may be specified in terms of access delay AD(p) for a given percentile p of the random access attempts, e.g., AD(0.8)<20 ms and AD(0.95)<40 ms implying that the access delay must be less than 20 ms and 40 ms for 80% and 95% of the random access attempts, respectively.
Furthermore, the access probability AP and access delay probability ADP are functions of two key factors, namely, the preamble detection miss probability and the contention probability. The detection miss probability DMP(m) at attempt m is the probability of a preamble, transmitted at attempt m, not being detected at the eNodeB. The contention probability CP is the probability that a UE is not granted access due to a preamble collision, i.e., that two or more UEs have chosen the same preamble (that are detected) during the same random access opportunity.
Uplink Control Signaling
To enable the base station eNB to acquire information from the UEs, different signaling mechanisms have been defined for LTE. The information can be UE internal, indicating for example buffer status or used power or it can be external indicating for example channel quality or received power.
L1 Control Signaling
Information with very tight time requirements are sent on the physical uplink control channel (PUCCH) or is multiplexed with uplink data transmissions on the physical shared channel (PUSCH). This transmission is fast but not as robust as it is not protected by any higher layer retransmissions. Examples of information that is sent with Layer 1 (L1) control signalling is HARQ ack/nacks, scheduling requests (SR) and channel quality indicators (CQI).
MAC Control Signaling
A more robust and dynamic way of sending uplink control information is by MAC control elements. Uplink transmissions on uplink shared channel (UL-SCH) consist of one or more MAC element. The elements are concatenated before channel coding and each has its own MAC header. The MAC control elements are slower than the L1 signaling since they may be retransmitted in case of decoding errors, but this also make them more robust against channel errors. Examples of information sent by MAC control elements are power headroom reports and buffer status reports. The type of control element is identified by a field LCID in the MAC header.
RRC Control Signaling
For slow reporting radio resource control (RRC) signaling is used. This reporting becomes very robust since it is protected by the Radio Link Control (RLC) Acknowledged Mode (AM) check, meaning that the data units are provided with sequence numbers, and received data units are acknowledged. This also makes this reporting rather slow, and the header overhead is rather large. But for slow reporting with a high number of bits it is a flexible and robust reporting. Measurements on neighbor cells, either on E-UTRAN or on other radio access technologies, are sent by RRC control signaling.
The flexible random access procedure in LTE is associated with a great number of parameters that affect the performance. Some of these parameters can be either set offline once and for all, or based on measurements in the eNodeB. Because the setting of the parameters affects the performance it is desired to provide a method and an apparatus enabling an efficient setting of the parameters. In order to enable efficient setting the performance needs to be observed.
Hence there exist a need for an improved method and apparatus for observing performance related to Random Access in an LTE system.